Device for automatically, or semiautomatically, changing the length of stroke of the grating advance mechanism in welding machines for welding wire gratings

ABSTRACT

In a machine for welding wire gratings, a device for automatically or semiautomatically changing the stroke lengths of the grating advance mechanism for the purpose of manufacturing gratings with different crosswire spacings, comprising a hydraulic motor for adjusting the grating advance mechanism to different stroke lengths having an electromagnetically operated servo valve in the delivery duct thereof which controls the operation of the motor, an actual value sensor forming part of a comparison and amplifier circuit and measuring the stroke length to which the grating advance mechanism has been adjusted by the motor, a plurality of desired value-setting devices preadjustable to desired stroke lengths, a relay switch individually connecting each one of the desired value-setting devices to the comparison and amplifier circuit for comparing the value to which each of the desired value-setting devices has been adjusted with the actual value of the stroke length, the comparison and amplifier circuit having an output fed to actuate the electromagnetically operated servo valve in such a way that the motor readjusts the actual stroke length of the grating advance mechanism to the value to which the desired value-setting devices have been adjusted, an electronic control device for actuation and deactuation of the relay switch according to a preselectable cycle, each of the relay switches remaining actuated for a preselectable number of advance strokes of the grating advance mechanism.

United States Patent [7 2] Inventors Heinz Sommeregger;

Josef Pohacker; Karl Schnlder, all of Graz, Austria [21] Appl. No. 799,427

[22] Filed Feb. 14, 1969 [45] Patented Oct. 5, 1971 [73] Assignee EVG Entwicltlunggsund Verwertungs- Gesellschalt m.lJ.H. Styria, Austria [32] Priority Feb. 16, 1968 [3 3 Austria [31] A 1499/68 [54] DEVICE FOR AUTOMATICALLY, OR

SEMIAUTOMATICALLY, CHANGING THE LENGTH OF STROKE OF THE GRATING ADVANCE MECHANISM IN WELDING MACHINES FOR WELDING WIRE GRATINGS Primary ExaminerMaynard R. Wilbur Assistant Examiner.leremiah Glassman Attorney-Ernest F. Marmorek ABSTRACT: In a machine for welding wire gratings, a device for automatically or semiautomatically changing the stroke lengths of the grating advance mechanism for the purpose of manufacturing gratings with different crosswire spacings, comprising a hydraulic motor for adjusting the grating advance mechanism to different stroke lengths having an electromagnetically operated servo valve in the delivery duct thereof which controls the operation of the motor, an actual value sensor forming part of a comparison and amplifier circuit and measuring the stroke length to which the grating advance mechanism has been adjusted by the motor, a plurality of desired value-setting devices preadjustable to desired stroke lengths, a relay switch individually connecting each one of the desired value-setting devices to the comparison and amplifier circuit for comparing the value to which each of the desired value-setting devices has been adjusted with the actual value of the stroke length, the comparison and amplifier circuit having an output fed to actuate the electromagnetically operated servo valve in such a way that the motor readjusts the actual stroke length of the grating advance mechanism to the value to which the desired value-setting devices have been adjusted, an electronic control device for actuation and deactuation of the relay switch according to a preselectable cycle, each of the relay switches remaining actuated for a preselectable number of advance strokes of the grating advance mechanism.

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ATTORNEY PATENTEU OBI 5197i SHEET 4 BF 4 INVENTORS ATTORNEY DEVICE FOR AUTOMATICALLY, OR SEMIAUI'OMATICALLY, CHANGING THE LENGTH OF STROKE OF THE GRATING ADVANCE MECHANISM IN WELDING MACHINFS FOR WELDING WIRE GRATINGS In grating-welding machines longitudinal wires are unreeled continuously from drums, and transverse wires, each cut to length, are welded transversely across the longitudinal wires, the welds made at the crossing point. The transverse wires are positioned and welded in place at desired intervals along the longitudinal wires. In most of the known grating-welding machines the already welded part of the grating is advances intermittently, the longitudinal wires moving forwards intermittently. With the grating stationary, and therefore the longitudinal wires stationary, a transverse wire is positioned across the longitudinal wires and welded to them at the crossing points by electric-resistance welding, whereupon the already welded part of the grating is advanced through the desired distance, a fresh transverse wire is positioned across the longitudinal wires, are welded to them, whereupon the grating is advanced again and so on. The spacing of the transverse wires, that is to say the distance between two neighboring transverse wires, is determined by the distance of advance, or stroke, of the mechanism which advances the grating. To change the spacing of the transverse wires it is necessary to change the stroke of the grating advance mechanism.

Grating-welding machines are known which are equipped with devices which allow the transverse wire spacing to be changed without interrupting the operation of the machine. This is done by changing the stroke length of the grating advance mechanism.

For example, the Austrian Pat. specification No. 186,940 describes a grating advance mechanism whose length of stroke can be changed continuously, that is to say steplessly, within limits determined by the particular construction, by rotating a toothed segment through a certain angle. The toothed segment is usually rotated by a hydraulic servosystem.

In a previous arrangement for automatically changing the transverse wire spacing in welded-wire gratings, there are mounted on the hydraulically driven shaft which rotates the toothed segment several cam discs whose angular positions relative to the shaft are adjustable. The angular movement of each cam disc is detected by a sensor which follows the contour of the cam. A programmed selector selects a particular sensor, and the position of this sensor, as it follows the contour of its cam disc, determines the transmission of control pulses to the hydraulic servosystem. The control pulses are transmitted by purely mechanical means, through a system of linkages. The desired stroke length of the grating advance mechanism is selected by adjusting the angular position of the cam disc on the drive shaft.

However this purely mechanical adjustment device has the disadvantage, in the first place, that the adjustment is comparatively coarse. Furthermore the mechanism is subject to rather rapid wear, and consequently the adjustment is not reliably reproducible. Finally, the process of adjustment, to give a different stroke length, is rather cumbersome because it means changing the angular position of a cam disc mounted on the drive shaft. The cam disc is often rather inaccessible. and it must be loosened. adjusted in position and then secured to the shaft again.

The object of the present invention is to provide an adjustment device of the general kind in question here, which is considerably more accurate and less subject to wear and which, in particular. allows the stroke length to be changed in a much simpler and more convenient way than has hitherto been possible.

The device according to the invention for automatically, or semiautomatically, changing the stroke of the grating advance mechanism in grating-welding machines. preferably on the basis of a preset program, for the purpose of manufacturing wire gratings having different desired transverse wire spacings. is characterized by the fact that an adjustment device which influences the stroke length of the grating advance mechanism is connected to a measured value transmitter whose indicated value corresponds to the existing position of the adjustment device, and in that there are several desired value transmitters, each of which can be preset to a particular desired value, the arrangement being such that at any given instant one of the desired value transmitters can be connected, preferably according to a preset program, together with the measured value transmitter to a comparison circuit whose output signal, representing the difference between the measured and the desired values, is fed to a servocontrol mechanism which changes the position of the device influencing the stroke lengths so as to make the measured value equal to the desired value.

Thus a device according to the invention always has a single measured valve transmitter, and a number of desired value transmitters equal to the number of different transverse wire spacings which it is desired to obtain.

Both the measured value transmitter and the desired value transmitters can if desired operate on an analog basis, for example they can be potentiometers operated in the known way. Alternatively if desired transmitters can be used operating on a digital basis, for example the measured value transmitter can take the form of a perforated plate controlling a ray of light reaching a photocell, in which case a digitalto-analog converter is preferably used before the signals are compared.

The selection of the particular desired value transmitter to be used, at a given instant, in cooperation with the measured value transmitter, is effected by actuating a relay switch. This can be done either by hand, or preferably by a programmed controller.

The programmed controller for actuating the relay switches, which activate the individual desired value transmitters, can if desired use perforated strips or magnetic ribbons. Preferably however there is used, for controlling the program, a counting device whose output terminals are connected to several preselection switches connected to storage units and wire-spacing selectors, these devices being arranged in such a way that as soon as the machine has completed the number of strokes previously set on the active preselection switch, the counting device inactivates the storage unit connected to this particular preselection switch, and activates the nest storage unit.

Further characteristics of the invention will be derived from the following description of examples, with the help of the drawings in which FIG. 1 represents diagrammatically the entire control apparatus.

FIG. 2 illustrates the method used for comparing the output voltages of the measured value and desired value transmitters, in the form of potentiometers, in order to derive a controlling quantity for actuating the servocontrol mechanism.

FIG. 3 represents an clectrooptical measured value transmitter, and a control circuit in which digital values are com pared and a controlling quantity derived.

FIG. 4 represents an electromagnetically actuated servo valve, which controls a hydraulic actuator motor, in regard to both speed and direction of rotation, in response to signals arriving from the electric control circuit.

FIG. 5 represents an electric circuit for selectively activating tree different desired value transmitters, in response to a preset program.

Referring first to FIG. I, an electric motor 1 drives an oil pump 2, which delivers oil to an oil duct 12 containing an electromagnetic servo valve 3 and an oil motor 4. A pressure control valve 16in a bypass duct 12a, cooperating with the pump 2, keeps the oil pressure in the oil circuit constant. The electromagnetic servo valve 3 is capable of reversing the flow of oil to the oil motor 4, or interrupting the flow of oil.

The oil motor 4 drives, through a shaft 17 a worm drive 5, a shaft 6 which itself drives, through a pinion 13, or if necessary through a reduction gear. the toothed segment 14 of the driving mechanism for advancing the wire grating. To the shah 6 there is connected a measured value transmitter 7, so that there is a positive and unambiguous correlation between the angular position of the shaft and the position of the measured value transmitter. The measured value transmitter 7, which detects the instantaneous angular position of the shafi 6, can be connected selectively. according to choice, with any one of three desired value transmitters 8a, 8b, 8c, the connection being effected by one of the tluee relay switches a, 10b, ltlc. these switches preferably having fully enclosed electric contacts immersed in an inert gas.

It should of course be understood that although three desired value transmitters and three relay switches are mentioned here, as an example, there can in practice by any desired number.

The measured value transmitter and the desired value transmitters can if desired be potentiometers, in which case the sliding contact of the measured value transmitter is moved directly by the shafi 6, in dependence on the angular position of the shaft 6. The sliding contact of the desired value transister potentiometer is adjusted by hand, this potentiometer preferably having a scale calibrated to show the transverse wire spacing in units of length.

in an alternative version of the invention the measured value transmitter is a digital counter, as represented in FIG. 3. The shaft 6 is connected to a perforated plate 38, either directly or through a multiplying gear. to the effect that the perforated plate rotates through a greater angle than the shaft 6. Whenever a hole of the perforated plate 38 comes in line with a light source 37, a ray of light falls on a photocell 39. situated on the other side of the perforated plate, whereupon the photocell 39 delivers a signal to a digital counter 40. The signal is converted into an analog voltage in a digitaltoanalog converter 46, and the resulting analog voltage is compared with the voltage delivered by one of the preset desired value transmitters 8a, 8b, 8c.

Alternatively the measured value transmitter can if desired by an electromagnetic pulse transmitter.

Furthermore the desired value transmitters can if desired take the form of digital-presetting devices cooperating with a digital-to-analog converter 45.

As a still further alternative then: can be used on the desired value side of the apparatus digitaLpresetting devices whose delivered digital signals are converted into analog voltages, whereas on the measured value side the apparatus uses analog quantities all the way through. Finally, the opposite arrangement to this can if desired by used.

The process of comparison, whereby the measured value is compared with the desired value, is effected in the circuits shown in FIGS. 2 and 3. Assuming. for example that the relay switch 10a is closed, the measured value transister 7 and the desired value transmitter 80 are both connected to the amplifier 9.

HO. 2 represents the two transmitters in the form of a measured value potentiometer 7 and a desired value potentiometer 8a. Let us assume, to begin with. that the sliding contact 18 of the desired value potentiometer Ba has been preset into a position corresponding to a particular. desired transverse wire spacing. The sliding contact 19 of the measured value potentiometer 7 assumes a position corresponding to the instantaneous angular position of the shaft 6. Let us assume that the two sliding contacts 18 and I9 deliver different voltages. These two voltages are both fed to a symmetrical-branched amplifier 9, where they are both amplified and then fed, with opposite polarity. to two electromagnetic coils 20, 200.

With reference to FIG. 4, the two coils 20 and 20:: contain in common a single iron core 25. which is attracted by both the coils. Ii the voltage is the same in the two coils. the iron core remains stationary in its neutral position. halfway between the two coils. On the other hand if the voltages on the two coils are difl'erent, the iron core is pulled towards the coil carrying the higher voltage.

The two ends of the iron core 25 are connected to a mechanical linkage system 26, 27 and 26a, 270. Of this linkage system. the two levers 27 and 1270 are pivoted at 49 and 49a. The free ends of the pivoted levers 27 and 270 are equipped with small control plates 47 and 47a situated near the outlet openings 30, 30a of the central drilling 29 of a differential piston 48, which forms part of an electromagnetic servo valve 3.

Oil is fed under presure by a pump 2 through an inlet opening 28 into the central drilling 29 of the diflerential piston 48, which has two throttling constrictions 50 and 50a. The oil escapes under pressure through the outlet openings 30 and 30a. At each end of the differential piston 48 the central drilling 29 is connected by drillings 35, 35a to two annular chambers 36 and 360 between the piston and the valve housing. The two annular chambers 36 and 36a contain oil under pressure supplied by the pump 2.

Let us assume that the measured value potentiometer 7 delivers a higher voltage over its sliding contact than that delivered by the desired value potentiometer 80. These voltages picked off by the two sliding contacts are amplified in the branched amplifier 9, the amplified voltages being fed to the coils 20, 20a. The coil 20 therefore carries a higher voltage than the coil 20a, and consequently the iron core 25 moves towards the coil 20.

The movement of the iron core 25 brings the control plate 47 closer to the outlet opening 30, whereas the control plate 47a moves away from the outlet opening 30a. in the central drilling 29 of the differential piston 48 there therefore occurs a pressure drop between the two ends of the piston. with the result that the pressure in the annular chamber 36 becomes higher than that in the annular chamber 360. and consequently the piston is thrust towards the right, allowing oil to flow as indicated by the continuous lines in FIG. 4. The oil motor 4 is driven in rotation, in the appropriate direction. as already described above, rotating the shaft 6 and the toothed segment 14 of the grating advance mechanism.

Due to the rotation of the shaft 6, the sliding contact 19 of the measured value transmitter 7 moves in a direction which decreases the voltage difference between the two transmitters. In order to obtain the most rapid adjustment and the quickest response, the branched amplifier 9 is constructed to have the steepest possible characteristic curve, so that even a comparatively small voltage difference between the two transmitters, corresponding to a difference of only a few millimeters (approximately 5-7 mm.) between the measured transverse wire spacing and the desired spacing, produces the highest voltage difference on the output terminals of the amplifier 9, this being enough to displace the iron core 25 as far as its limiting position, as determined by one of the two mechanical stops. which are not shown in the drawing.

For example, suppose that it is desired to change the transverse wire spacing from 300 mm. to I00 mm. The oil motor 4 runs at full speed to bring the spacing down from 300 mm. to 107 mm. measured value. From 107 mm. down to I00 mm., that is to say during the last 7 mm. of readjustment, the voltage difference between the coils 20 and 20a is gradually reduced to zero, the iron core 25 and the difi'erential piston 48 gradually returning to their middle positions, the speed of the oil motor 4 at the same time gradually falling to zero. This arrangement allows the transverse wire spacing to be changed so quickly that the period of time occupied by the changeover is less than the period during which the grating remains stationary. It is therefore possible to change over from one spacing to the next between the welding of one transverse wire and the welding of the next.

if the apparatus uses digital transmitters, the control process proceeds as described above, signals being delivered by the digital-to-analog converters 45, 46 to the coils 20. 200. In this case the measured value transmitter can if desired be a perforated plate, as already mentioned and as shown in FIG. 4, the perforated plate being interposed between a light source and a photocell.

Alternatively the measured value transmitter can if desired be a circular plate equipped with metal sectors which move between the poles of a magnet. Whenever a metal sector passes across the magnetic pulse, the magnetic field is disturbed and a pulse is delivered. The pulses are counted in a digital counter and then converted into analog voltages in a digital-to-analog converter. The resulting analog voltages are then compared with the voltage delivered by a desired value transmitter. In this case the measured value transmitter operates by induction.

The selection of the particular desired value transmitter which is to be connected for the voltage comparison is effected by the relay switches a. 10b, 10:, in response for example to the commands of a perforated ribbon or a magnetic ribbon, according to a preset program.

However, a particularly favorable and preferred selector system is represented diagrammatically in FIG. 5. A counter 11 counts the strokes of the grating advance mechanism. beginning with one, the output terminals of the counter 11 are connected to a series of preselection switches 21. 23. 13. 24 (there can of course be any desired number ofthese switches). On each preselector switch is selected. by the operator. the number of transverse wires which are to be welded at a particular desired spacing.

in FIG. 5 the preselection switch 21 is set to the value 005, that is to say to live transverse wires. The preselection switch 22 is set to 10 transverse wires, the preselection switch 23 is set to 3 transverse wires and the preselection switch 24 is set to zero transverse wires, that is to say it is witched ofl' entirely. The switch 24 ensures that no transverse wires are welded at the spacing corresponding to this switch.

The counter and the preselection switches each have 30 decade-coded contacts, so that in this example, in which each preselection switch has three decimal places, each preselection switch can deliver a welding command for up to 999 transverse wires, to be welded at the same spacing. Among all the 999 possible contact combinations only one, this being the combination which has been preset on the preselection switch, allows electric current to flow from the counter over the preselection switch to one of the storage units 31, 32, 33, 34 connected to the preselection switches. Each preselection switch therefore functions as an electric switch point.

In this example, as soon as the counter has reached the count five" a control pulse is transmitted from the counter ll over the preselection switch 21 to the storage unit 31.

The storage units 31, 32, 33, 34 form an electronic relay chain whose function it is to open or block a current path, each blocking command being accompanied simultaneously by an opening command delivered to the neighboring storage unit.

The spacing selector switches 41, 42, 43, 44 allow each of the four storage units 31, 32, 33, 34 to be connected to any one of the three relay switches 10a, 10b, 10:.

For example let us assume that the storage unit 31 has just been connected. When the counter 11 has counted up to five," it delivers a signal which switches 06 the storage unit 31 and switches on the storage unit 32, over the cross connection between the two counters. The relay switch 10b, which is connected to the storage unit 31 over the spacing selector switch 41, therefore breakgcontact. The relay switch 100, which is connected to the storage unit 32 over the spacing selector switch 42, is closed. The desired value transmitter 80, as shown in FIG. 1. is connected to the circuit, delivering a voltage for comparison. The counter 11 at the same time returns to zero, and begins counting again. When the counter 11 has counted up to 10, the storage unit 32 is connected over the preselection switch 22 to the counter 11, with the result that the storage unit 32 is switched off, at the same time switching the storage um! 33 on. A signal closes the relay switch 10:. connecting the desired value transmitter Us to the circuit. to provide a voltage for comparison.

When the counter it has again counted up to three. the storage unit 33 is switched offend the storage unit 34 switched on. However the preselection switch 14 is set to 000, and consequently the counter 11. which always returns to zero when the next storage unit is engaged, immediately witches off the storage unit 34 again, the storage unit 34 switch on the storage unit 31. The storage unit 34, is therefore ineffective, because the preselection switch 24 has been set to 000.

The individual components of the circuit, for example the counter, the preselection switches, the storage units and the like, are commercially available parts and have been described here only as far as has been considered necessary for clarifying the method of functioning of the system as a whole.

The switching system described above has the great advantage, compared to a control system using perforated ribhon or magnetic ribbon, that it is not necessary to provide easily torn ribbons. All the setting operations are performed directly on the machine by manipulations which can easily and reliably be effected by appropriately instructed personnel.

What we claim is:

1. lo a machine for welding wire gratings, a device for auto matically or semiautomatically changing the stroke lengths of the grating advance mechanism for the purpose of manufacturing gratings with difl'erent cross-wire spacings comprising a fluid motor for adjusting the grating advance mechanism to diflerent stroke lengths, an electromagnetically operated servo valve in the delivery duct of said fluid motor controlling the operation of said motor, an actual value sensor, said actual value sensor forming part of a comparison and amplifier circuit and measuring the stroke length to which the grating advance mechanism has been adjusted by said fluid motor, a plurality of desired value setting devices preadjustable to desired stroke lengths, relay switch means individually connecting each one of said desired value setting devices to said comparison and amplifier circuit for comparing the value to which each of said desired value setting devices has been adjusted with the actual value of the stroke length, said comparison and amplifier circuit having an output fed to actuate said electromagnetically operated servo valve in such a way that the fluid motor readjusts the actual stroke length of said grating advance mechanism to the value to which said desired value setting device has been adjusted, an electronic control device for actuation and deactuation of said relay switch means according to a preselectable cycle, each of said relay switch means remaining actuated for a preselectable number of advance strokes of the grating advance mechanism, wherein said electronic control device comprises a counting device and a number of preselection switches, said counting device and said preselection switches each having coded contacts, a number of storage units equal to the number of preselection switches and each one of said storage units being associated with one of said preselection switches, said storage units forming an electronic relay chain, each one of said storage units immediately ahead of it in the electronic relay chain and being inactivated by an impulse transmitted to it from said counting device by way of one of said preselection switches associated with said storage unit, a number of spacing selector switches equal to said number of preselection switches for selectively connecting any one of said storage units to any one of a number of relay switches, each one of said relay switches remaining activated as long as the storage unit connected to it remains actuated.

2. The combination according to claim 1, characterized in that said actual value sensor and the desired value setting devices are electric potentiometers.

3. The combination according to claim 1, characterized in that at least the actual value sensor is a digital system whose signals are fed to a counter and to a digital-to-analog converter which delivers an analog voltage for the comparison.

4. The combination according to claim 3, characterized in that the actual value sensor comprises a rotatable perforated disc whose angular position depends on that of the control device, the perforated plate being positioned between a light source and a photocell whose signals are fed to a digital counter.

5. The combination according to claim 3, characterized in that the actual value sensor is an inductive pulse transmitter 7. The combination according to claim 6. characterized by counter means for actuating the individual relay switches.

8. The combination according to claim 1, characterized in that said fluid motor is an oil motor, and said electromagnetically operated servo valve is itself controlled by a quantity derived by comparing the actual value with the desired value. 

1. In a machine for welding wire gratings, a device for automatically or semiautomatically changing the stroke lengths of the grating advance mechanism for the purpose of manufacturing gratings with different cross-wire spacings comprising a fluid motor for adjusting the grating advance mechanism to different stroke lengths, an electromagnetically operated servo valve in the delivery duct of said fluid motor controlling the operation of said motor, an actual value sensor, said actual value sensor forming part of a comparison and amplifier circuit and measuring the stroke length to which the grating advance mechanism has been adjusted by said fluid motor, a plurality of desired value setting devices preadjustable to desired stroke lengths, relay switch means individually connecting each one of said desired value setting devices to said comparison and amplifier circuit for comparing the value to which each of said desired value setting devices has been adjusted with the actual value of the stroke length, said comparison and amplifier circuit having an output fed to actuate said electromagnetically operated servo valve in such a way that the fluid motor readjusts the actual stroke length of said grating advance mechanism to the value to which said desired value setting device has been adjusted, an electronic control device for actuation and deactuation of said relay switch means according to a preselectable cycle, each of said relay switch means remaining actuated for a preselectable number of advance strokes of the grating advance mechanism, wherein said electronic control device comprises a counting device and a number of preselection switches, said counting device and said preselection switches each having coded contacts, a number of storage units equal to the number of preselection switches and each one of said storage units being associated with one of said preselection switches, said storage units forming an electronic relay chain, each one of said storage units immediately ahead of it in the electronic relay chain and being inactivated by an impulse transmitted to it from said counting device by way of one of said preselection switches associated with said storage unit, a number of spacing selector switches equal to said number of preselection switches for selectively connecting any one of said storage units to any one of a number of relay switches, each one of said relay switches remaining activated as long as the storage unit connected to it remains actuated.
 2. The combination according to claim 1, characterized in that said actual value sensor and the desired value setting devices are electric potentiometers.
 3. The combination according to claim 1, characterized in that at least the actual value sensor is a digital system whose signals are fed to a counter and to a digital-to-analog converter which delivers an analog voltage for the comparison.
 4. The combination according to claim 3, characterized in that the actual value sensor comprises a rotatable perforated disc whose angular position depends on that of the control device, the perforated plate being positioned between a light source and a photocell whose signals are fed to a digital counter.
 5. The combination according to claim 3, characterized in that the actual value sensor is an inductive pulse transmitter responding to the position of the control device and delivering signals to a digital counter.
 6. The combination according to claim 5, characterized in that each of the desired value setting devices, each of which can be preset independently of the others, cooperates with said relay switch for actuating said corresponding desired value transmitter which delivers a signal for comparison with the signal delivered by the actual value sensor.
 7. The combination according to claim 6, characterized by counter means for actuating the individual relay switches.
 8. The combination according to claim 1, characterized in that said fluid motor is an oil motor, and said electromagnetically operated servo valve is itself controlled by a quantity derived by comparing the actual value with the desired value. 